Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharging rate have been commercialized and widely used.
A carbon material has been mainly used as an anode active material of these lithium secondary batteries, and the use of lithium metal, a sulfur compound, a silicon compound, and a tin compound has also been in consideration. Also, lithium-containing cobalt oxide (LiCoO2) has been mainly used as a cathode active material, and in addition, the use of lithium-containing manganese oxide, such as layered structure LiMnO2 and spinel structure LiMn2O4, and lithium-containing nickel oxide (LiNiO2) has also been in consideration.
Since LiCoO2 has excellent various properties such as cycle characteristics, LiCoO2 has currently been widely used. However, LiCoO2 has low safety, is expensive as a raw material due to the resource limit of cobalt, and has limitations in being largely used as a power source for various applications such as electric vehicles. LiNiO2 may have difficulty in being used in an actual mass production process at a reasonable cost due to the characteristics according to the manufacturing method thereof, and lithium manganese oxides, such as LiMnO2 and LiMn2O4, may have poor cycle characteristics.
Thus, a method of using lithium transition metal phosphate as a cathode active material has recently been studied. Lithium transition metal phosphate is broadly classified into NASICON-structured LixM2(PO4)3 and olivine-structured LiMPO4, and is studied as a material having excellent high-temperature safety in comparison to typical LiCoO2. Currently, Li3V2(PO4)3 has been known among NASICON-structured compounds, and LiFePO4 and Li(Mn,Fe)PO4 have been most widely studied among olivine-structured compounds.
In particular, since LiFePO4, as a material having a voltage of about 3.5 V vs. lithium, a high bulk density of 3.6 g/cm3, and a theoretical capacity of 170 mAh/g, among the olivine-structured compounds has better high-temperature stability than cobalt (Co) and uses inexpensive iron (Fe) as a raw material, it is highly possible for LiFePO4 to be used as a cathode active material for a lithium secondary battery in the future.
However, since LiFePO4 has the following limitations, LiFePO4 has limitations in commercialization.
First, since LiFePO4 has low electrical conductivity, the internal resistance of a battery may increase when the LiFePO4 is used as a cathode active material. As a result, polarization potential increases when a battery circuit is closed, and thus, battery capacity may be reduced.
Second, since LiFePO4 has a lower density than a typical cathode active material, there is a limitation that the energy density of the battery may not be sufficiently increased.
Third, since an olivine crystal structure in a state in which lithium is deintercalated is very unstable, a movement path of lithium ions is obstructed by a portion in which lithium is removed from the surface of a crystal. Thus, the intercalation/deintercalation of lithium may be delayed.
Therefore, a technique has been proposed in which a discharge capacity is increased by decreasing the moving distance of lithium ion by reducing the diameter of the olivine crystals to a nanoscale level.
However, in the case that an electrode is prepared by using olivine particles having a fine particle size, exfoliation from a cathode collector may be facilitated due to a spring back phenomenon, and a large amount of a binder must be used in order to reduce the exfoliation.
However, in the case in which the large amount of binder is used, resistance may increase and voltage may decrease. Also, the mixing time of a cathode active material composition may increase, and thus, process efficiency may decrease.